Sintered ores that are used as a material for blast furnaces are generally manufactured by undergoing a treatment process of a sintering material as shown below. As shown in FIG. 1, at first, iron ores M1 with a particle size of 10 mm or less, an SiO2-containing material M2 including silica rock, serpentine rock or nickel slag, a CaO-containing limestone base powdery material M3 such as limestone, and a solid fuel type powdery material M4 that is a heat source such as powdery coke or anthracite are mixed together with an appropriate amount of water followed by granulating by means of a drum mixer 4, and thereby granulated products called as pseudo-particles are formed.
The blended material including the granulated products is charged on a pallet of a Dwight-Lloyd sintering machine with an appropriate thickness of, for instance, 500 to 700 mm, a solid fuel at a surface thereof is ignited, after the ignition, the solid fuel is combusted with air sucking downwardly, and, by the combustion heat, the blended sintering material is sintered to be a sintered cake. The sintered cake is pulverized and screened, and thereby sintered ores having a particle size larger than a predetermined diameter are obtained. On the other hand, ores having a particle size smaller than the predetermined diameter are returned and used again as the sintering material.
The reducibility of the sintered ore product thus manufactured is, as so far pointed out, a factor that largely controls an operation particularly of the blast furnace. Ordinarily, the reducibility of the sintered ores is defined according to JIS M8713 (JIS: Japanese Industrial Standard, hereinafter referred to as JIS), and here the reducibility of the sintered ores is denoted with JIS-RI.
As shown in FIG. 2, there is a positive correlation between the reducibility (JIS-RI) of the sintered ores and a gas utilization factor (ηco) in the blast furnace, and furthermore, as shown in FIG. 3, there is a negative correlation between the gas utilization factor (ηco) in the blast furnace and a fuel ratio. Accordingly, the reducibility (JIS-RI) of the sintered ores is in an excellent negative correlation with the fuel ratio through the gas utilization factor (ηco) in the blast furnace. As a result, when the reducibility of the sintered ores is improved, the fuel ratio in the blast furnace decreases.
The gas utilization factor (ηco) and the fuel ratio are defined as follows.Gas utilization factor (ηco)=CO2 (%)/(CO (%)+CO2 (%))
Here, CO2 (%) and CO (%) each mean volume % in a furnace top gas of the blast furnace.Fuel ratio=(amount of coal+coke used (kg))/amount of pig (1 ton)
Furthermore, the cold strength of the sintered ore product thus manufactured is also an important factor for securing the ventilation in the blast furnace. In the individual blast furnaces, the lower limit of the cold strength is set and operated. Accordingly, the sintered ores preferable for the blast furnace are one that is excellent in the reducibility and high in the cold strength. In Table 1, the reducibility and the tensile strength of four main ore textures that constitute the sintered ores, that is, calcium ferrite (CF): nCaO.Fe2O3, hematite (He): Fe2O3, calcium silicate containing FeO (CS): CaO.xFeO. ySiO2, and magnetite (Mg): Fe3O4 are shown. As shown in Table 1, one that is high in the reducibility is hematite (He) and one that is high in the tensile strength is calcium ferrite (CF).
The desired sintered ore structure that the invention intends is, as shown in FIG. 4, one that has calcium ferrite (CF) high in the strength generated on a surface of the sintered ore and has hematite (He) high in the reducibility selectively generated toward the inside of the sintered ore. Calcium silicate (CS) low in the reducibility and the strength should not be formed as far as possible.
However, so far, as mentioned above, the iron ore M1, SiO2-containing material M2, limestone base powdery material M3 and solid fuel type powdery material M4 are simultaneously mixed and granulated. Accordingly, as shown in FIG. 5, in the pseudo particle structure, there are powdery ores, lime, and coke mixed in the surroundings of coarse seed ores. Accordingly, in the sintered ore structure obtained by the sintering, four ore textures of hematite (He), calcium ferrite (CF), calcium silicate (CS) and magnetite (Mg) are mingled.
In this connection, so far, various methods have been tried so as to produce much calcium ferrite (CF) and hematite (He). For instance, calcium silicate (CS) is produced a lot when the sintering is carried out at high temperatures. Accordingly, in Japanese Unexamined Patnt Application Publication No. 63-149331, a technology is proposed in which powdery iron ore, together with a binder and limestone, is granulated followed by coating powdery coke that is a heat source on a surface to improve the combustibility of coke, this is sintered at low temperatures and thereby the reducibility is improved.
However, according to the conventional method proposed in Japanese Unexamined Patnt Application Publication No. 63-149331, since CaO and SiO2 in the iron base material or SiO2 base material are in proximity to each other, calcium silicate (CS) is inevitably produced much. Accordingly, in many cases, a structure mainly including calcium ferrite (CF) and hematite (He) is not necessarily obtained.
Furthermore, Japanese Unexamined Patnt Application Publication No. 63-69926 proposes a technology in which after powdery iron ore and/or returned ore is mixed, to the mixed powdery iron ore and/or returned ore limestone, powdery coke and auxiliary raw materials such as scale and silica rock are added to form pseudo particles, thereby the powdery coke can be deposited much on an outer periphery portion of the pseudo particle, thereby the combustion speed of the powdery coke is accelerated, resulting in shortening the combustion time.
However, according to the conventional method proposed in Japanese Unexamined Patnt Application Publication No. 63-69926, since limestone and silica rock in the auxiliary raw materials are present together, calcium silicate (CS) weakest in the tensile strength is produced a lot, resulting in fragile sintered ores low in the strength.
Still furthermore, Japanese Unexamined Patnt Application Publication No. 11-241124 discloses a method of manufacturing low SiO2 sintered ores in which, after iron ore powder, returned ores, part or whole of calcined lime and limestone and part or whole of SiO2 source material are mixed and granulated by use of a primary mixer, powdery coke divided from another system and the slug source such as silica rock and lime are added to the mixed and granulated material followed by granulating by use of a secondary mixer, thereby on a surface portion of the granulated particle a layer of powdery coke and slug source is formed, and thus obtained material is sintered to obtain the low SiO2 sintered ores.
However, according to the technology disclosed in Japanese Unexamined Patnt Application Publication No. 11-241124, in an exterior coating portion of the granulated particle (that is, one corresponding to a pseudo-particle of the invention), the low SiO2-containing material is likely to enter. Thereby, calcium silicate (CS) lowest in the tensile strength among constituent ores of the sintered ore as shown in Table 1 is formed, resulting in lowering the Chatter Index or the Tumbler Index that denotes the cold strength. Furthermore, inside of the granulated particle, the material partially containing limestone enters; accordingly inside of the sintered ore, not only hematite (He) high in the reducibility but also calcium ferrite (CF) inferior in the reducibility to hematite (He) and calcium silicate (CS) much inferior in the reducibility to the hematite (He) are formed; as a result, a dramatic improvement effect in the reducibility cannot be obtained.
Furthermore, Japanese Unexamined Patnt Application Publication No. 61-163220 discloses a pre-treatment method of the sintering material. In the method, a sintering material, in which, pellet is mixed while the humidity therein is controlled, is mixed by use of a primary mixer and subsequently, powdery coke is added to the humidity-controlled and granulated substance which is then subjected to rollintg granulation by use of a secondary mixer.
However, according to the technology disclosed in Japanese Unexamined Patnt Application Publication No. 61-163220, since the material containing limestone enters inside of the pseudo particles, inside of the sintered ores, not only hematite (He) high in the reducibility but also calcium ferrite (CF) inferior in the reducibility to hematite (He) and calcium silicate (CS) remarkably inferior in the reducibility to the hematite (He) are formed. Accordingly, not only the dramatic improvement effect of the reducibility cannot be obtained, but also in the outside of the sintered ore where the cold strength has to be secured, calcium silicate (CS) lowest in the tensile strength among the ingredient ores of the sintered ore is formed, resulting in decreasing in the Chatter Index or the Tumbler Index that denotes the cold strength.
As disclosed in Japanese Unexamined Patnt Application Publication Nos.61-163220, 63-69926 and 11-241124, in the pre-treatment method or manufacturing method of the sintering material in which with the primary and secondary mixers, the mixing and granulation are carried out, fundamentally the primary mixer performs the mixing and granulation mainly consisting of the mixing of the sintering material, and thereafter, the secondary mixer carries out the granulation. When there are the primary and secondary mixers like this (when there are two mixers in total), ordinarily, for the mixing and granulation of the sintering material in the primary mixer, substantially 120 seconds are secured, and for the granulation in the secondary mixer, substantially 180 seconds are secured.
Furthermore, as to the additional coating of powdery coke and limestone, in Japanese Unexamined Patnt Application Publication No.2002-285250, an applicant the same as the present invention discloses a manufacturing method of a sintering material that the present invention intends to obtain. That is, there is proposed a granulation method in which by additionally coating powdery coke and limestone, so-called three-layer pseudo particles are obtained. The additional coating of powdery coke and limestone intends to deposit an auxiliary raw material including the additionally coated powdery coke and limestone on a surface of the pseudo particle. Thereby, to the pseudo particle with a first layer of a coarse particle and a second layer of fine particles surrounding the coarse particle, on a surface layer of the pseudo particle a third layer rich in powdery coke and limestone is formed, and thereby the reducibility JIS-RI value of the sintered ore can be improved.
However, even in the Japanese Unexamined Patnt Application Publication No. 2002-285250, it was found that when powdery coke and limestone were additionally coated in the course of granulation, in the drum mixer, other than an action of forming the pseudo particles owing to rolling of the drum mixer, breaking down of the pseudo particles was repeated in the course of the rolling; in this breaking down process, powdery coke and limestone were contained inside of the pseudo particles; as a result, powdery coke and limestone could not be coated on the surface of the pseudo particles.
Furthermore, in Japanese Unexamined Patnt Application Publication No. 2002-285250, powdery coke and limestone are additionally coated by inserting a belt conveyer into a drum mixer to add.
However, the aditional coating method described in Japanese Unexamined Patnt Application Publication No. 2002-285250, in particular a method that uses a belt conveyer has the following disadvantages. That is, deposit adhered to an inner wall of the drum mixer in the course of the granulation of the material for sintering falls down on the belt conveyer to adhere to and deposit on the belt conveyer. In order to remove the accretion and deposit, it takes a lot of labor. Furthermore, in some cases, a driving part of the belt conveyer is damaged and an operation is interrupted. Still furthermore, when the accretion on the belt conveyer becomes too large, the accretion comes into contact with the inner wall of the drum mixer, or the belt conveyer is bent owing to a weight of the accretion to come into contact with the inner wall of the drum mixer. It was found that such a contact of the inner wall of the drum mixer and the accretion gave rise to a large damage onto the inner wall of the drum mixer, other than the interruption of the operation was caused, there was a large problem also from the viewpoint of safety.
Furthermore, Japanese Unexamined Patnt Application Publication No. 58-189335 discloses another additional coating method. According to the method, over a region from an intermediate portion in a direction in which material in the drum mixer flows to an ore exhaust side (exhaust side), an air stream is used to inject and add from the exhaust side.
However, according to the method disclosed in Japanese Unexamined Patnt Application Publication No. 58-189335, equipment expense for an air stream generator that additionally coats auxiliary raw materials, apparatus for transferring additional coating additives and injection equipment becomes enormous. Furthermore, to a portion that is inside of the drum mixer of the injection equipment, the accretion falls from the inner wall of the drum mixer or dust powder adheres to apparatus portion, resulting in disturbing a smooth operation. Furthermore, according to the method, since the additional coating material is injected and added toward a charge side of the drum mixer by the air stream, the additional coating material is widely scattered within the drum mixer and reaches to the charge side of the drum mixer. As a result, a problem is caused in that since such auxiliary raw material that were scattered up to the charge side are contained in the sintering material in the course of granulation in the drum mixer, the intention of depositing the additional coating auxiliary raw material on the pseudo particle surface cannot be realized.
Still furthermore, a still another additional coating method is proposed in Japanese Unexamined Patnt Application Publication No. 2002-20820. According to the method, in a predetermined region on a sintering material charge side in the drum mixer, by making use of an air stream, a binder consisting limestone powder and hydrated lime and so on is dispersed and added.
However, even according to the method disclosed in Japanese Unexamined Patnt Application Publication No. 2002-20820, since a portion of the apparatus that projects the additional coating auxiliary raw material is always inside of the drum mixer, dust powder (calcined lime and so on) in the drum mixer adheres and solidly sticks to the portion of the apparatus and disturbs the operation. Accordingly, the maintenance operation that periodically interrupts the operation and pulls the portion of the apparatus out to remove the accretion is necessary. However, in the maintenance operation, since it is difficult to pull out the portion of apparatus, the maintenance operation takes a lot of time.
Furthermore, similarly to the Japanese Unexamined Patnt Application Publication No. 58-189335, the additional coating auxiliary raw material is widely scattered in the drum mixer and reaches up to the charge side of the drum mixer. The auxiliary raw material scattered up to the charge side is taken in the sintering material in the course of the granulation by the drum mixer, accordingly there is a problem in that the additional coating auxiliary raw material cannot be deposited onto the pseudo particle surface.
The present invention was carried out to overcome the above-mentioned conventional problems and intends to provide a manufacturing method of a material for sintering that can improve the cold strength and the reducibility of the sintered ore and apparatus therefor. The inventive manufacturing method includes, as a pre-treatment of a process of manufacturing sintered ore, without necessitating huge apparatus, forming pseudo particles by granulating iron ore M1 and SiO2-containing material M2 separately from limestone base material M3 and solid fuel type material M4; and selecting a time to additionally coat limestone base material M3 and solid fuel type material M4 to gradually form pseudo particles, and thereby manufacturing sintered ore having a structure in which a layer rich in the limestone base material M3 and solid fuel type material M4 is formed on a surface portion of the pseudo particle, calcium ferrite (CF) high in the strength is generated on the surface of the sintered ore, and on the other hand toward the inside of the sintered ore, hematite (He) high in the reducibility is selectively formed.
In the present invention, iron ore of material for sintering includes coarse, powdery iron ore and returned ore that is again utilized as the sintering material, and with these generically referring as iron ore, the invention will be explained.